Projection screens



May 16, 1961 A. l. MIHALAKIS PROJECTION SCREENS 2 Sheets-Sheet 2 OrigmalFiled Nov. 23 1951 INVENTOR. 5%; I MlYia/J/Zlii' BY We, M -M fi w UnitedStates Patent M PROJECTION SCREENS Agis I. Mihalakis, Buffalo, N.Y.,assignor, by mesne assignments, to William T. Snyder, South Lancaster,Mass.

Original application Nov. 23, 1951, Ser. No. 257,691,

now Patent No. 2,804,801, dated Sept. 3, 1957. Divided and thisapplication Aug. 27, 1957, Ser. No. 683,437

14 Claims. (Cl. 88-283) The present invention relates to optical screensof the type suitable to receive an image formed by projection apparatusand to present the image to observers. This application is a division ofmy application Serial No. 257,691, filed November 23, 1951, now PatentNo. 2,804,801 of September 3, 1957.

It is one of the primary objects of the present invention to provide aprojection screen which not only provides optimum utilization of theavailable image energy, but utilization of such image energy, even ofcomparatively low level, under very unfavorable conditions ofextraneous, nondesirable light. -It will be understood that suchextraneous or ambient light, while increasing the intensity level of theprojected image, for perceptional purposes does not sufficiently raisethe intensity of the highlight portions of the image, thus decreasingthe contrast of, and blotting out the projected image. With asufficiently high image intensity, screens designed according to thepresent invention remain operative under extremely unfavorableconditions, such as broad daylight falling upon the screen.

Another object of the invention is to provide a screen of the above typewhich reflects into a given observation field as defined by the lateraland vertical viewing angles, a substantially constant average imageintensity, so that the screen retains uniform brilliance over the entirearea, whether it is viewed directly in the optical axis of projector andscreen system or from a side within the field of observation.

A further object of the invention is to provide projection systems ofsimple design, and easy manufacture and operation, which permitsatisfactory utilization of illumination levels for which conventionaldevices have proved to be inoperative or requiring cumbersome andexpensive arrangements.

In accordance with one aspect of my invention, a screen for viewing,from within a field of observation, an image projected on the screenfrom an object region, comprises a plurality of juxtaposed substantiallynondif fusing elementary optical systems either all convex or concave oralternatingly convex and concave, and together forming the total screensurface and being curved such as to present elementary imagesessentially only to the field of observation, and to direct lightincident on the screen from without the field of observation to anypoint of that field essentially only at intensities which are at thatpoint lower than the intensity directed by the screen to such point fromthe object. a

In another aspect, the elementary surfaces have boundaries constitutingstops and curvatures determining image areas such that the relativevalues of the stops and curvatures define elementary object and imagefield angles which together admit to a field of observation essentiallyall light from an object while excluding an essential amount of lightincident from without the fields of observation. Thus the elementaryimages effected by the elementary surfaces are presented essentiallyonly to the field of observation while light incident on the screen2,984,152 Patented May 16, 1961 from without that field is essentiallyexcluded from the field of observation. In accordance with a preferredembodiment of the invention, the elementary optical systems arealternately convex and concave and merge to form a continuouslyundulated screen surface so that the above-mentioned field angles are atboundary regions where the respective convex and concave curves merge.

A further important feature of the invention is elementary surfaces withdouble curvature either negative or positive or both, at substantiallyall points of the screen surface with the exception of theabove-mentioned boundary regions.

Another important feature of the invention is the shaping of theelementary optical systems in such a manner that the above-mentionedfield angles differ in lateral and vertical directions to conform to agiven field of observation with different dimensions in thesedirections, so that the image light energy available from a given objectis directed mainly into that field and a practically minimum amountthereof into space outside thereof, while an essential part ofextraneous light, that is light coming from outside the field ofobservation of such dimensions, is directed into space outside of thatfield, which has the above-mentioned effect of providing at any point ofthe field of observation an image intensity which is considerably higherthan the intensity that reaches the same point from such extraneouslight sources. In this manner the projected image retains under mostpractical circumstances contrast sufficient for distinct perceptionregardless of the intensity of extraneous light, whether strong director diffuse artificial or daylight. The brillance of screens according tothe invention is under most practical conditions high enough to preventcontrast reduction and hence image deterioration other than coming fromthe projected object.

In a further more specific aspect, the invention contemplates elementaryoptical elements of certain dimensions, which have proved to beparticularly beneficial and which will specifically be described hereinwith reference to their actual performance.

While certain features of the present invention relate mainly toobservation of the image from the side facing the projector or otherobject, others apply as well to back projection screens which transmitthe image light or other useful light from one surface of the screen tothe other, with the field of observation on the opposite side from theobject. Such features include the configuration of lens elements whichreplace the mirror elements of opaque or front projection screens, andthe spatial correlation of such elements. In certain instances theoptical elements according to the invention will refleet as well asrefract light incident thereon, with useful results derived from theirpeculiar configuration and correlation.

Other objects, aspects, and features, in addition to those contained inthe above short statement of the nature and substance including some ofthe objects of the invention, will appear from the herein presentedexposition of its basic theoretical principles so far as they can beascertained at the present time and from the following description ofseveral typical embodiments thereof illustrating its novelcharacteristics. This description refers to drawings in which Fig. 1 isa diagrammatical median section through a screen according to theinvention;

Fig. 2 is a diagram indicating the first-order theory of the fieldangles and stops of mirrors of the type herein employed;

Fig. 3 is a diagrammatic cross section through several elementaryoptical elements of a screen according to the invention, illustratingits performance with regard to the formation of elementary images and ofthe presentation of these images;

4 is diagrams illustrating undesirable dead areas;

Figs. to 7 illustrate the actual configuration of a practlcal embodimentof a screen according to the invention, in plan view and sections onlines 66 and 77 respectively; and

Fig 8 is a diagrammatical representation of screens according to theinvention in environment determining its structure.

Figs. 1 and 2 show a screen S of conventional overall d1mens1onsarranged for projection thereon of an image produced by conventionalprojection apparatus p comprising a light source 21, a condenser 22, adiapositive such as a film strip 23, and a projector lens system 24-. Itwill be understood that the projector as a whole or the diapositive 23defines an object region containing this embodiment as an object thediapositive which can be considered to consist of object elements a(Fig. 2) that are by the lens system 24 imaged on correspondingelementary areas a of screen S which are defined by elementary opticalsystems s, such as the curved mirrors indicated in Figs. 1 and 2.

Fig. 1 shows, in a simplified cross section through part of screen S, byway of example convex and concave elementary mirrors si and sj whichtogether constitute a continuously undulated screen surface. Opticallyspeaking, convex and concave elements are equivalent for purposes of theinvention, as will further appear from the following discussion. Whilealternately convex and concave elements offer important advantages inaccordance with one of the aspects of the invention, other features ofthe invention can be accomplished with only convex or only concaveelements otherwise constructed and arranged according to the invention.

The screen S can be made of suitable sheet metal such as brass or steel.Optically less desirable metal screens can be coated, if desirable, withanother metal as for example nickel, chromium, aluminum or silver. Thescreen body can be made with essentially equidistant front and rearsurfaces, or it can be solid with a flat rear surface. The screen canalso be made of dielectric material such as polymerized plasticcompounds shaped in accordance with the invention and coated with ametal layer applied to the reflecting layer. A protective layer ofsuitable optical properties can be applied over such coating. Such aprotective layer is sometimes also desirable in the case of solid metalscreens. In every case, the optically eflective surface must be polishedor otherwise smoothed to the highest possible degree commensurate withthe material and purpose at hand, in order to render it non-scattering.

Fig. 2 shows a single screen mirror element, in this instance a convexelement si. The optical properties of the elements are for presentpurposes sufliciently described and determined by the axis e, ordinarilybut not necessarily parallel to system axis z, radius r, object fieldangle a/2, and image field angle a'/2. As well known, these valuesdetermine, together with a given object and observer the stops andentrance and exit pupils and windows respectively; these again determinethe field covered by the optical element in terms of a/ 2, and theemerging rays in terms of a/ 2. For purposes of the invention asembodied in the examples given hereinbelow, the angle a is quite largeand its definite value immaterial, so that only angle a is indicated.

' Since the dimensions of the mirror elements can be freely selectedwithin the psychophysiological limitations to be referred to below, themirror element configuration as a Whole can be fitted to the given fieldor fields of observation. Thus, an optimum amount of light energyavailable can be directed and spread into the field .of ob servation, soto speak from the object (in this case the diapositive), andconcentrated therein with minimum loss and therefore optimum brillianceof the elementary image as perceived by the observer within the field ofobservation. Furthermore, any light from a source outside the field ofobservation is deviated into space outside that field, so that it cannotinterfere with the projected image. If this outside light is coming fromoutside of and undesirable in the field of observation, such as director indirect artificial or daylight, the contrast of the object such as adiapositive will be preserved and indeed improved as compared to thatobtainable with conventional screens which do not reject such extraneouslight and therefore often obliterate the image by lowering the contrastto physiologically undesirable degree. If the outside light isoperatively desirable such as that of a second projector, the two (or ifdesired more) fields of observation can be kept apart. This latterembodiment of the invention will be described more in detailhereinbelow.

For purposes of the present example with considerable distance betweenscreen and projector, it can be assumed that the mirror axes areparallel; it will be understood however that, for more nearly equalobject and image distances from the imaging system, the mirrorconfiguration can be determined for inclined mirror axes and nonparallelincident pencils, in accordance with well known principles.

Possible differences in average image distance of convex and concaveelements respectively, are of second order importance especially in viewof the fact that the images extend axially, so that they are forpractical purposes within a plane formed by the mirror boundaries.

The elementary images are subject to considerable aberration but themirror elements are designed to furnish images which are as concentratedas possible. This does not necessarily mean that they must be small inall dimensions, but they are designed in accordance with the principlesof reflecting surfaces to furnish optimum brilliance. This brilliance isconsiderably higher than that of extraneous light which might bereflected into the field of observation by unavoidable irregular oruncontrollable regions of the reflecting screen. This brilliance isfurther higher than that of light which usually might originate in thefield of observation.

The distorted real or virtual elementary images can be considered toconstitute light sources which are as to intensity and color modulatedby the corresponding object element. The mirror profiles need not beperfect so long as they are highly polished and have the abovementionedfield properties; aberration is favorable for averging the detail of theobject element. I observed that small but high intensity images fuse aswell as larger less intense images at similar separations.

The peculiar choice of elementary image configuration and spacing isbased, in accordance with the invention, on the concept not to attemptmatching or registering of the border regions of comparativelyundistorted elementary images, but to provide small image points whichare highly concentrated transversely of the respective mirror elementeven so as to act as comparatively intense sources of light modulated asto wave length and amplitude by the object, such asa transparency in aprojector. It is thus not the regularity of the elementary images whichcontrols essentially continuous and well defined presentation of theentire image, but the effective separation and brilliance relation ofthe elementary point images. Such fusion is governed by difierentpsychophysiological principles than those applying to the matching ofadjacent low intensity patterns. It was found that in accordance withthis principle, the elementary systems can be larger than it washeretofore supposed to be feasible, with the ensuing mechanicaladvantages including those of ease of manufacture, possibility of higherpolish, and possibility of better control of shape. 7 Since according tothis concept the pattern of each individual image point is irrelevantand therefore aberration and distortion is beneficial rather thandetrimental, the optical elements can be shaped and surfaced purely witha view to providing a maximum energy output into the field ofobservation, by judicious choice of surface properties and object andimage field angles together with field stop defining boundaries or rims.

The above characteristic features are illustrated in Fig. 3 which showsseveral elementary mirrors s, with transversely concentrated real andvirtual images q extending axially, a ray pencil 1i, and a plane Ithrough the boundary regions of the elementary mirrors.

It will now be evident that comparatively little detrimental light isreflected into a chosen field of observation so that, as initiallystated, a screen composed of mirror elements according to the inventionreflects such detrimental light into the field of observationessentially only at intensities which are at any one point of that fieldlower than the intensity of the image.

The optimum mirror element configuration for a given purpose can beeasily arrived in accordance with the principles of the invention ateither theoretically as indicated above, or empirically. The followingpractical embodiments have given the satisfactory performance in dicatedin connection therewith.

The screen shown in Figs. to 7 has the following dimensions indicated inthese figures.

A screen constructed according to Figs. 5 to 7, 22" wide and 19 high,placed at a distance of 14 feet from a standard 35 mm., 500 wattprojector, with the projector approximately normal to the screen, isdetermined by a field of observation having the following approximatedimensions indicated in Fig. 8.

m1: 1 1 feet m2'=15 inches Angle ah=107.20

Angle av=8.20

It will be understood that the front edge distance of the field ofobservation is approximate and that the practically useable field beginsat a greater distance from the screen. Needless to say the field extendsin the rear theoretically into infinity and is practically limited inthat direction by architectural or other considerations.

With as many as ten flood lights with reflectors of 500 watts each,shining directly on the screen, placed in a solid bank at 3 feethorizontal distance from the screen and 3 feet vertical distance fromthe projector axis, with the screen according to Figs. 5 to 7 receivingan image from the above projector, an image of excellent contrast wasobtained on the screen according to Figs. 5 to 7, whereas the image on aconventional beaded screen was extinguished by excess illuminationreceived from the floodlights. The following comparative values in footcandles indicate the respective performances, these values having beenmeasured in the projector axis at distances m3, etc.

Projector and extraneous sources effective Unmodulated light from theprojector alone without film was measured at 52 PC at the center of thescreen.

These values are essentially the same in lateral points at the abovedistances, close to the borders of the object field as shown in Fig. 8,with abrupt decrease of image contrast beyond these borders.

This test is confirmed by the following readings taken at the samepoints but with the flood lights extinguished and the projector inoperation as before. This test also indicates the improved efliciency ofscreens according to the invention, apart from the rejection ofdetrimental extraneous illumination.

Projector alone efiective l4 l5 l6 l7 0. 0. 85 0. 80 0. 75

The beneficial effect of screens according to the invention is stillfurther illustrated by a test during which the screens used for theabove tests, were in the same setup and environment, affected by theflood lights only, with the projector turned ofl. The flood lightintensity measured at 500 PC at the center of the screens. Under theseconditions, the light reflected from the screen according to theinvention was measured 0.5 PC at 1 foot from the screen while the beadedscreen measured at PC at the same point.

The above screens are made with substantially nonscattering surfaces ofbrass, polished to mirror smoothness, and if desired, chromium or nickelplated depending upon requirements of color. selectivity. It was foundthat the highly polished brass surface is satisfactory for manypurposes.

Screens of the above type can be manufactured in various ways. Someshapes can be milled by conventional shop methods. Others, such as thataccording to Figs. 5 to 7, are preferably made as follows. A mastermodel is first made by hand or machine, to a scale large enough topermit fairly exact incorporation of the predetermined curvatures. Thismodel is then reduced to actual scale by copying on a three dimensionalengraving machine of conventional design employing the pantographprinciple, in material suitable for use of the actual scale copy as amatrix. The screens are then pressed from the matrix according toconventional practice. Such a surface has been engraved into a knurlingtool and such surfaces can be knurled into rolls, or engraved intorolls, creating, by conventional methods, a male and female pair throughwhich sheet material can be rolled.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents which fall within the scope of the appended claims.

What is claimed is:

1. An optical screen for presenting to a field of observation the imageof an object region projected on said screen, comprising an essentiallycorrugated surface which is substantially everywhere curved in the samesense with respect to said field of observation, each corrugation beingmodified to provide a curvature undulating from concave to convex in adirection lengthwise of the corrugations in a plane perpendicular to theplane of the screen and to provide a ridged curvature intersecting saidplane, the concave and convex undulations having a greater radius ofcurvature than the radius of curvature of the corrugations, providing ahighly directional imaging surface composed of a plurality of elementaryoptical surfaces each having a curved rim and imaging cross sectionalcurvatures, said elementary optical surfaces being arranged in rows withundulating contours and being substantially contiguously joined at saidrims which joined rims form two intersecting series of aperture lines,one series being continuously curved and lying in a plane that isessentially parallel to a plane tangential to said contours, the secondseries of aperture lines being recurrently curved to form ridges at theaperture lines of the first series whereby the surface areas within eachrow are essen- 7 tially optically wholly controllable so that the highlydirectional screen presents said image to said field of observation withhigh intensity.

2. Screen according to claim 1, wherein said elementary optical surfacesare in said plane perpendicular to the plane of the screen alternatelyconvex and concave and merge to form a continuously undulated screensurface within said continuously curved aperture lines,

3. An optical screen for presenting to a field of observation the imageof an object region projected on said screen, comprising an essentiallycorrugated surface which is substantially everywhere curved in the samesense with respect to said field of observation, each corrugation beingmodified to provide a curvature undulating from concave to convex in adirection lengthwise of the corrugations in a plane perpendicular to theplane of the screen and to provide a ridged curvature intersecting saidplane, the concave and convex undulations having a greater radius ofcurvature than the radius of curvature of the corrugations, providing ahighly directional imaging surface composed of a plurality of elementaryoptical surfaces each having a rim curvature and imaging cross sectionalcurvatures for conjointly producing elementary stops and image fields ofcorresponding configurations, said elementary optical surfaces beingarranged in rows with contours continuously varying through crosssections wherein they are substantially contiguously joined at said rimsto form one series of continuously curved aperture lines with adjacentrows joined at ridges and constituting a second series of ridgedaperture lines intersecting said first series at said ridges, wherebyeach elementary surface presents to its image field an elementary imageof optimum intensity, which image fields can be determined by selectionof said rim and imaging curvatures to conform to a predetermined fieldof observation.

4. An optical screen for presenting to a field of observation the imageof an object region projected on said screen, comprising an essentiallycorrugated surface which is substantially everywhere curved in the samesense with respect to said field of observation, each corrugation beingmodified to provide a curvature undulating from concave to convex in adirection lengthwise of the corrugations in a plane perpendicular to theplane of the screen and to provide a ridged curvature intersecting saidplane, the concave and convex undulations having a greater radius ofcurvature than the radius of curvature of the corrugations, providing ahighly directional imaging surface composed of a plurality of elementaryoptical surfaces each having a rim curvature and imaging cross sectionalcurvatures for conjointly producing rectangular elementary stops andimage fields, said elementary optical surfaces being arranged instaggered rows with alternatingly convex and concave contourscontinuously varying with points of inflection through one series ofcross sections lengthwise of said corrugation and varying similarly butin alternatingly opposite sense through a second series of crosssections lengthwise of said corrugation, within respective pairs of rimswith points of inflection, said series of cross sections being parallelto each other and being substantially contiguously joined at said rims,said rims and said points of inflection, respectively, forming tworectangularly intersecting series of curved aperture lines, whereby thesurface areas within each row are essentially optically curved in thesame sense throughout, and optically wholly controllable, and eachelementary surface presents to its image field an elementary image ofoptimum intensity, which image fields can be made rectangular byselection of said rim and imaging curvatures to conform to a rectangularfield of observation.

5. An optical screen for presenting to a field of observation the imageof an object region projected on said screen, comprising an essentiallycorrugated surface which is substantially everywhere curved in the samesense with respect to said field of observation, each corrugation beingmodified to provide a curvature'undulating from concave to convex in adirection lengthwise of the corrugations in a plane perpendicular to theplane of the screen and to provide a ridged curvature intersecting saidplane, the concave and-convex undulations having a greater radius ofcurvature than the radius of curvature of the corrugations, providing ahighly direction reflective imaging surface composed of a plurality ofelementary curved mirrors each having a curved rim and imaging crosssectional cur vatures for conjointly producing elementary stops andimage fields of corresponding configurations, said elementary mirrorsbeing arranged in rows with undulating contours, and being substantiallycontiguously joined at said rims which joined rims form two intersectingseries of curved aperture lines, one scrim being continuously curved andlying in a plane that is essentially parallel to a plane tangential tosaid contours, the second series of aperture lines being recurrentlycurved to form ridges at the aperture lines of the first series wherebythe joining surface areas within each row are essentially opticallywholly controllable, and each elementary mirror presents to its imagefield an elementary image of optimum intensity, which image fields canbe determined by selection of said rim and imaging curvatures to conformto a predetermined field of observation.

6. A screen according to claim 5 wherein said elementary mirrors areelongate with an approximately 2.75 to 1.2 ratio of the maximumdimension to the minimum dimension.

7. A screen accordang to claim 6 wherein said maximum dimension is inthe order to magnitude of 0.275 inch.

8. Screen according to claim 5 wherein said imaging curvatures havealternate approximate apex radii of 0.03 inch and 0.055 inchesrespectively.

9. A screen according to claim 5 wherein said rows are staggered withsaid continuously curved aperture lines forming an essentially linearpattern of minimal length.

10. An optical screen for presenting to a field of observation the imageof an object region projected on said screen, comprising an essentiallycorrugated surface which is substantially everywhere curved in the samesense with respect to said field of observation, each corrugation beingmodified to provide a curvature undulating from concave to convex'in adirection lengthwise of the corrugations in a plane perpendicular to theplane of the screen and to provide a ridged curvature intersecting saidplane, the concave and convex undulations having a greater radius ofcurvature than the radius of curvature of the corrugations, providing ahighly directional reflective imaging surface composed of a plurality ofelementary curved mirrors each having curved rims and imaging crosssectional curvatures for conjointly producing rectangular elementarystops and image fields of corresponding configurations, said elementarymirrors being arranged in staggered rows with points of inflectionbetween alternatingly convex and concave contours lengthwise of saidcorrugation located within, and substantially contiguously joined atsaid rims, said rims and said points of inflection, respectively,forming two intersecting series of curved aperture lines one of whichseries is continuously curved and lies in a plane that is essentiallyparallel to a plane tangential to said contours, the second series ofaperture lines being recurrently curved to form ridges at the aperturelines of the first series whereby the joining surface areas within eachrow are essentially optical curved in the same sense throughout, andoptically wholly controllable, and each elementary mirror presents toits image field an elementary image of optimum intensity, which imagefields can be made rectangular by selection of said rim and imagingcurvatures to conform to a rectangular field of observation.

11. An optical screen for presenting to a field of observation the imageof an object region projected on said screen, comprising an essentiallycorrugated surface which 7 is substantially everywhere curved in thesame sense with modified to provide a curvature undulating from concaveto convex in a direction lengthwise of the corrugations in a planeperpendicular to the plane of the screen and to provide a ridgedcurvature intersecting said plane, the concave and convex undulationshaving a greater radius of curvature than the radius of curvature of thecorrugations, providing a highly directional reflective imaging surfacecomposed of a plurality of elementary curved mirrors each havingcircularly curved rims and circular imaging cross sectional curvaturesfor conjointly producing rectangular elementary stop and image fields ofcorresponding configurations, said elementary mirrors being arranged instaggered rows with points of inflection between alternatingly convexand concave contours lengthwise of said corrugation, and said elementarymirrors being located within, and substantially contiguously joined atsaid rims, said rims and said points of inflection, respectively,forming two intersecting series of curved aperture lines one seriesbeing continuously curved and lying in a plane that is essentiallyparallel to a plane tangential to said contours, the second series ofaperture lines being recurrently curved to form ridges at the aperturelines of the first series whereby the surface areas within each row areessentially optically curved in the same sense throughout, and opticallywholly controllable, and each elementary mirror presents to its imagefield an elementary image of optimum intensity, which image fields canbe determined by selection of said rim and imaging curvatures to conformto a predetermined field of observation.

12. A projection screen for optimum utilization by reflection of lightimpinging thereon, comprising an essentially corrugated surface, eachcorrugation being modified to provide a curvature undulating fromconcave to convex in a direction lengthwise of the corrugations in aplane perpendicular to the plane of the screen and to provide a ridgedcurvature intersecting said plane, the

concave and convex undulations having a greater radius of curvature thanthe radius of curvature of the corrugations, providing a plurality ofoptical elements which together constitute a highly directional imagingsurface, said optical elements being substantially everywhere curved inthe same sense with respect to said impinging light, such that theboundaries between adjacent elements are formed by the points ofcurvature change from one element to the next and constitute twointeresting series of lines, one series being continuously curved andlying in planes which intersect each other in lines that are parallel tothe imaging surface as a whole and the other series having reversalpoints and lying in planes which intersect each other in parallel linesthat are parallel to the imaging surface as a whole and oblique to saidintersection lines of said first series, whereby said surface isessentially being substantially everywhere curved in the same sense withrespect to said impinging light, wholly directional and image formingwithout flat and scattering areas, and said lines constitute theaperture boundaries of each one of said elements.

13. Screen according to claim 12 wherein each of said optical elementsis curved in relation to its boundaries such that the viewing zone inwhich the projected image becomes visible is of rectangularconfiguration.

14. Screen according to claim 12 wherein said planes of said firstseries of lines coincide such as to constitute a single plane that isessentially parallel to the imaging surface as a whole, and said planesof said second series of lines are parallel and intersect said imagingsurface as a whole at substantially right angles.

References Cited in the file of this patent UNITED STATES PATENTS2,804,801 Mihalakis Sept. 3, 1957

